This class of anthelmintics are difficult to formulate. For example, the commercially available injectable antiparasitic agent based on an aqueous co-solvent formulation containing ivermectin, water and an organic co-solvent, has been shown to result in some precipitation of the ivermectin at the injection site, with the possibility of irritation to the animal, and possible loss of effectiveness as an anthelmintic. In pour-on formulations, water miscible formulations are diluted if there is rain after treatment.